An understanding of the CHF mechanisms is useful for the user of empirical correlations and prediction methods and for devising means to avoid the occurrence of the phenomenon. Let us consider CHF mechanisms and their regions of operation for concurrent flow conditions. A large number of alternative mechanisms for CHF have been proposed but four concepts which appear to have been reasonably well established experimentally are illustrated in Figure 7 and areas follows:

Formation of hot spot under growing bubble (Figure 7.7 (a)). Here, when a bubble grows at the heated wall, a dry patch forms underneath the bubble as the micro-layer of liquid under the bubble evaporates. In this dry zone, the wall temperature rises due to the deterioration in heat transfer. When the bubble departs, the dry patch may be rewetted and the process repeats itself. However, if the temperature of the dry patch becomes too high, then rewetting does not take place and gross local over heating (hence CHF) occurs. This mechanism was proposed, for instance, by Kirby (1966, 1967).

Near-wall bubble crowding and inhibition of vapor release (Figure 7.7 (b)). Here, a "bubble boundary layer" builds up on the surface and vapor generated by boiling at the surface must escape through this boundary layer. When the boundary layer becomes too crowded with bubbles, vapor escape is impossible and the surface becomes dry and overheat giving rise to burnout. This mechanism is discussed, for instance by Tong et al. (1972).

Dryout under a slug or vapor clot. In plug or slug flow, the thin film surrounding the large bubble may dry out giving rise to localized overheating and hence burnout. Alternatively, a stationary vapor slug may be formed on the wall with a thin film of liquid separating it from the wall; in this case, localized drying out of this film given rise to overheating and burnout. This mechanism has been investigated, for instance, by Fiori and Bergles (1968) (Figure 7.7 (c)).

Film dryout in annular flow (Figure 7.7 (d)). Here, in annular flow, the
liquid film dries out due to evaporation and due to the partial entrainment to the liquid in the form of droplets in the vapor core. This mechanism is discussed in more detail below.

One should note that each of these mechanisms has a direct relation to the two-phase flow regime in which CHF occurs; e.g., bubble crowding in subcooled nucleate boiling, vapor clotting in slug flow, or film dryout in annular flow. Thus CHF is fundamentally a condition where liquid cannot rewet the heater wall because of the rate of vapor production impeding the liquid flow back to the hot surface. As the flow regime changes (e.g., bubbly, slug, annular) due to variations in mass velocity, pressure or geometry the particular mechanism which prevents the liquid to rewet the heater surface changes, but the basic principle remains the same. One might actually again use quality as a method to describe this. Figure 7.8 shows a conceptual picture of these four mechanisms as a function of G and . Once again the quality contains the effect of many of these mentioned variables. One different situation which might occur is the case of counter current flow and the CHF mechanism associated with it. In this case liquid drains down the channel wall due to gravity as vapor, produced along its length, flows upward (Figure 7.9). In this situation one holds up the liquid flow into the whole channel due to the production of vapor along the whole channel length. This liquid holdup can be likened to a flooding phenomenon at the entrance of the channel causing film dryout lower in the channel. Oscillatory behavior of the CHF dryout may also occur in this situation.